Characterizing Unforced Multi-decadal Variability of ENSO: A Case Study with the GFDL CM2.1 Coupled GCM

Large multi-decadal fluctuations of El Nino-Southern Oscillation (ENSO) variability simulated in a 4,000-year pre-industrial control run of GFDL CM2.1 have received considerable attention due to implications for constraining past and future changes in ENSO. We evaluated the realism and mechanisms of this low-frequency ENSO modulation through analysis of the extreme epochs of CM2.1 as well as through the use of a linearized intermediate-complexity model of the tropical Pacific, which produces reasonable emulations of observed ENSO variability. Analyses of ENSO in the CM2.1 and the linear model suggest that in the control run of CM2.1, intrinsically-generated epochs of extreme ENSO variability arise from transient nonlinearities or multiplicative noise, with tropical Pacific mean state changes serving to damp the intrinsically-generated ENSO modulation. Like most coupled General Circulation Models, CM2.1 suffers from large biases in its ENSO simulation, including overly strong ENSO variance. We find that the overly strong ENSO variance in CM2.1 directly contributes to its strong multi-decadal modulation through broadening the distribution of epochal variance, which increases like the square of the long-term variance. It is shown that the overly strong ENSO variance in CM2.1 results in multidecadal ENSO modulation that is twice that of a linear system with observed (1951-2001) ENSO variance. These results suggest that the true spectrum of unforced ENSO modulation is likely substantially narrower than that in CM2.1, and thus detection of forced changes in ENSO from paleoclimate records or observations may be more easily attained than that implied by CM2.1.